WINSEM2013 14 CP0530 03 Mar 2014 RM01 Lecture 20 PPE Feed Water Treatment

7/25/2019 WINSEM2013 14 CP0530 03 Mar 2014 RM01 Lecture 20 PPE Feed Water Treatment

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Boiler Feed Water Treatment:

Need:

Boiler makeup of 1.5 to 2% of total flow is required tobe replenished for loss of water.

Fittings

Bearings o er ow own

Escape with condensable gases in deaerator, turbine glands,

and other causes


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feeding it in to the boiler 1. Prevention of hard scale formation on the heating

surfaces

2. Elimination of corrosion . ontro o carry-over to e m nate epos t on on super

heater tubes, and

4. Prevention of silica de osition and corrosion dama e

to turbine blades


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e Raw water s t ere ore F rst pre-treate an t en

demineralized.

For once throu h boilers and boilin water nuclear

reactors, which require high water purity, a condensatepolishing system is used to further polish the water.

Raw water contains a variety of impurities, such as

(1). Suspended solids and turbidity

2 . Or anics

(3). Hardness- Salts of calcium and magnesium

(4). Alkalinity- bicarbonates, carbonates and hydrates

. , , (6). Silica

(7). Dissolved gases- O2, CO2

The extent of pre-treatment depends on the source of raw water.


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Externa reatment:

First step of pre-treatment of boiler is clarification- water is

chlorinated to revent biofoulin of e ui ment

The suspended solids and turbidity are coagulated byadding special chemicals- Aluminum sulphate, Al2(SO4)3

,

and Agitated

The coagulated matter settles at the bottom of the clarifier

If the turbidity of clarified effluent is high, positive filtration

is needed.

Both gravity filters and pressure type filters are used.

The pressure difference across the filtering medium in an indication

of solid accumulation


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External Treatment:

When it reaches a given limit, the solids are removed from

t e e y ac was ng.

The filtration by activated carbon can absorb organics and

remove residual chlorine from chlorination rocess.

The dissolved salts of calcium and magnesium - Hardness

All scale forming impurities are calcium and magnesiumin the form of carbonates, bicarbonates, chlorides and

sulphates.

. The Softening of water can be done by lime-soda process,

phosphate process, zeolite process and demineralization.


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Lime soda Process:

n me so a so en ng , a c um an magnes um

salts are removed using Lime(calcium hydroxide)

When this process is carried out at normal raw

water tem erature Cold Process- softenin

When this process is carried out at near boiling

oint Hot Process- softenin

Hot process is preferred for boiler water treatment


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Lime soda Process:

e represen a ve reac ons are

Ca(HCO3)2 + Ca(OH)2 = 2CaCO3 + 2H2O

g 2 + a 2 = g 2 + a 2

MgSO4 + Ca(OH)2 = Mg(OH)2 + CaSO4 CaSO4 + Na2CO3 = CaCO3 + Na2SO4

CaCl2 + Na2CO3 = CaCO3 + 2NaCl

The Calcium carbonate and magnesium hydroxideare insoluble in water and settles to the bottom of

e vesse


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Hot phosphate softening:

e ca c um an magnes um ar ness s remove

by using phosphate and caustic soda

3 4 2

hydroxide are precipitatedo

and above

phosphate softening


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Hot phosphate reactions are:

a 3 a a2 3 2

a 3 + a3 4 = a3 4 + a2 3

3 2 2 2 3 2

4 3 4 3 4 2 2 4


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Sodium Zeolite Softening:

a er can e so ene y pass ng roug a e

of sodium zeolite, the cations of calcium and

CaCO3 + NaZ = CaZ + Na2CO3

MgSO4 + Na2Z = MgZ + Na2SO4

Ca(HCO3)2 + Na2Z = CaZ + Na2(HCO3)2


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The softening capacity of the bed can be

CaZ + 2NaCl = Na2Z + CaCl2

MgZ + 2NaCl = Na2

Z + MgCl2

e eo e so en ng s no ea or o ow ng

reasons:

.on zeolites

.


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The Zeolite softening is not ideal for following

3. There can be silica gain in water from the zeolite

. ,

corrosive.

use of evaporators may be more effective.


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Hydrogen Zeolite softening:

en wa er con a n ng ca c um, magnes um an

sodium ions is passed through a hydrogen zeolite,

bicarbonate, sulphate, chloride and nitrate radicals

are converted to their respective acids. (Ca) (Ca)

M HCO + H Z = M Z + 2H CO

(Na2) (Na2)


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(Ca) (Ca)

g 4 2 g 2 4

(Na2) (Na2)

When the hydrogen zeolite becomes exhausted, its ac was e an regenera e w ac .

After being rinsed, it is ready for use again. The

. Mgz + H2SO4 = H2Z + MgSO4

a 2 4 2 a 4


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Anion Exchangers:

n on exc angers can remove e an on e

chlorides , sulphates and nitrates (acid form)

materials which absorb them.

The reactions are

2HCl + RCO3 = RCl2 + H2CO3

+ = +

2HNO3 + RCO3 = R(NCO3)2 + H2CO3

.


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Anion Exchangers:

en e ac c wa er s spraye n a s ower o

expose large surface area, the carbon dioxide is gas

H2CO3 CO2 + H2O

,

regenerated by backwashing with soda ash

+ = +

RSO4 + Na2CO3 = RCO3 + Na2SO4


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Dem nera z ng P ant:

The process of removing dissolved solids in water by

.

Two types of resins, cation and anion are used

hydrogen ion is exchanged for cations calcium,

magnesium and sodium. The anion resins adsorb the anions chlorides, nitrates

and sulphates.

Both ion exchange process are reversible, and resinsare restored to their original form by regeneration


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Dem nera z ng P ant:

A typical Demineralizing plant consists of a cation

, ,

silica adsorber in series

In the de asifier carbon dioxide as is removed b

aeration.

Silica in water is very detrimental at high pressure. Magnesium hydroxide is often used to adsorb silica

from water.

The dissolved solids from makeup water is removed byreverse osmosis or electrodyalysis.


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High quality water is produced by demineralizing systems,

but the treated water can pickup impurities due to

condenser leakage from the circulating water and iron and

copper from pipelines.

condensate through large demineralizing vessel, called

mixed bed units, which contain both cation and anionresins.

The resins not only remove dissolved salts, but also acts as

Power plants using once through boilers and nuclear

reactors require high quality of water and use condensate

polishing system.

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WINSEM2013 14 CP0530 03 Mar 2014 RM01 Lecture 20 PPE Feed Water Treatment